BH-condensate systems could also emit GWs. Bosonic fields centered with a Kerr BH can form bound states like the hydrogen atom. Especially, the bosonic cloud could gain sufficiently large mass and angular momentum by superradiance mechanism such that their spacetime disturbance can produce observable GWs. There exist numerous research works on various bosons, including spin-0 [1]}, [2]}, [3]}, [4]}, [5]}, [6]}, [7]}, [8]}, [9]}, [10]}, [11]}, [12]}, [13]}, [14]}, [15]}, [16]}, [17]}, [18]}, [19]}, [20]}, [21]}, [22]}, spin-1 [23]}, [24]}, [25]}, [26]}, [27]}, [22]}, [29]}, [30]}, [31]}, [32]}, [21]}, [34]}, [35]}, [36]} and spin-2 [37]}, [38]} fields. It has been shown that the superradiant process happens when the frequency \(\omega \)  of a bosonic wave satisfies \(0<\text{Re}(\omega )<m\Omega _H\) , where \(m\)  is the azimuthal index of the instability mode and \(\Omega _H\)  is the horizon angular velocity. From conservation laws, the energy and the angular momentum are extracted from the rotating BH at the center. We refer interested readers to Ref. [39]} for comprehensive discussions on superradiance.
